Optical fibers are flexible, transparent devices which can be used to transmit information or images by means of optical signals. An optical fiber generally consists of a glass or plastic core surrounded by a cladding, where the refractive index of the core is higher than that of the cladding. The cladding protects the core-cladding interface from contaminants and contact with adjacent fibers, any of which could cause losses due to leakage of the signal propagating in the fiber. Light propagates through the fiber by means of multiple total internal reflections induced by the cladding which occur at the core-cladding interface.
Optical fibers are used in a variety of applications as a means of delivering power or transmitting images to a remote location. Optical fibers may be coupled to a laser to transmit information to a photodetector by modulating the intensity of the laser light. They may also be doped and used as an amplification medium, where an optical signal induces the stimulated emission of photons from the excited atoms of the dopant which has been energized by light of a different wavelength than that of the signal being amplified.
In situations where optical power is delivered by means of an optical fiber, it is often important to know how much power is being transmitted by the fiber, either at a point along its length, or at its end. This measurement is usually made by placing a photodetector and a device which indicates the intensity of the detected signal (which is calibrated so that it acts as a power meter) at the fiber end, or using optical elements (lenses, half-silvered mirrors) placed at the fiber end to enable the beam to be sampled from a position off of the fiber axis.
These methods of measuring the intensity of the optical signal traveling through a fiber (and from that determining the optical power) are based on directly measuring the total or fractional intensity at the output end. While useful, this approach has several drawbacks. It is intrusive and may unnecessarily complicate the optics at the output end. Also, it is often desirable to measure the injected power at the input end of the fiber to allow adjustment of the input end optics, and in some situations it is not feasible to introduce a detector at this position. Another drawback of such methods is that the sampling of the optical power may reduce it below the level at which it is intended to be used. In addition, depending upon the application for which the fiber is being used, there may be insufficient room at the fiber end for the placement of diagnostic equipment.
Diagnostic methods have also been applied to determine the optical power passing through an optical fiber at an intermediate position along the length of the fiber. One such method is based on measuring the intensity of the light leaked from the sides of the fiber. However, this method has been shown to be inaccurate because the intensity which is measured was found to depend upon the injection geometry, i.e., the light measured is that injected into the fiber at high angles, and not the throughput power corresponding to light propagating along the fiber axis.
Another approach which can be used is to modify the core and cladding so as to induce additional leakage. This is accomplished by scoring the core, bending the fiber, or using index matching. In these cases it is again found that the rays injected at extreme launch angles are more likely to escape the fiber and be detected. Thus, measurements made under these conditions do not provide an accurate estimate of the power carried by the fiber.
What is desired is a means of measuring the intensity of a signal transmitted by an optical fiber, and hence determining its power, in a manner which provides an accurate representation of the intensity of the light propagating along the fiber axis, and which does not interfere with optics or other equipment placed at the ends of the fiber.